ByteSlice: Fix persisting ptr to std::moved SSO buffer

The Bug:
1. Construct `byte_slice.portion_` with `epee::span(buffer)` which copies a pointer to the SSO buffer to `byte_slice.portion_`
2. It constructs `byte_slice.storage_` with `std::move(buffer)` (normally this swap pointers, but SSO means a memcpy and clear on the original SSO buffer)
3. `slice.data()` returns a pointer from `slice.portion_` that points to the original SSO cleared buffer, `slice.storage_` has the actual string.

contrib/epee/src/byte_slice.cpp

@@ -133,10 +133,13 @@ namespace epee
 
   template<typename T>
   byte_slice::byte_slice(const adapt_buffer, T&& buffer)
-    : storage_(nullptr), portion_(to_byte_span(to_span(buffer)))
+    : storage_(nullptr), portion_(nullptr)
   {
     if (!buffer.empty())
+    {
       storage_ = allocate_slice<adapted_byte_slice<T>>(0, std::move(buffer));
+      portion_ = to_byte_span(to_span(static_cast<adapted_byte_slice<T> *>(storage_.get())->buffer));
+    }
   }
 
   byte_slice::byte_slice(std::initializer_list<span<const std::uint8_t>> sources)

tests/unit_tests/epee_utils.cpp

@@ -387,6 +387,29 @@ TEST(ByteSlice, Construction)
   EXPECT_FALSE(std::is_copy_assignable<epee::byte_slice>());
 }
 
+TEST(ByteSlice, DataReturnedMatches)
+{
+  for (int i = 64; i > 0; i--)
+  {
+    std::string sso_string(i, 'a');
+    std::string original = sso_string;
+    epee::byte_slice slice{std::move(sso_string)};
+
+    EXPECT_EQ(slice.size(), original.size());
+    EXPECT_EQ(memcmp(slice.data(), original.data(), original.size()), 0);
+  }
+
+  for (int i = 64; i > 0; i--)
+  {
+    std::vector<uint8_t> sso_vector(i, 'a');
+    std::vector<uint8_t> original = sso_vector;
+    epee::byte_slice slice{std::move(sso_vector)};
+
+    EXPECT_EQ(slice.size(), original.size());
+    EXPECT_EQ(memcmp(slice.data(), original.data(), original.size()), 0);
+  }
+}
+
 TEST(ByteSlice, NoExcept)
 {
   EXPECT_TRUE(std::is_nothrow_default_constructible<epee::byte_slice>());